TwoTwoWayDirectionalValve

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Description

This model describes a 2/2-way directional control valve with second order spool dynamics. The flow through the valve is described as laminar/turbulent flow through an orifice:

dp = pa.p - pb.p

phi = sign(dp) * Cd * A(sp) * sqrt( (2/rho) * abs(dp) ) + GLeak * dp;

Here Cd is the discharge coefficient which normally has a value between 0.55 and 0.7. A(sp) is the area of the orifice opening. A(sp) depends linearly on the spool position sp and varies between 0 (sp = 0) and the maximum area Amax (sp = 1). Gleak is the conductance of laminar leakage flow when the valve is closed. The relative opening of the spool valve is indicated by sp. For a closed valve the spool position (sp) is equal to zero and for an open valve the spool position (sp) is equal to 1.

In the neutral spool position (sp = 0) the valve is just closed. A positive overlap indicates that the spool must travel a certain distance before the valve opens.

A negative overlap indicates that the valve is already open in the neutral position.

The overlap is indicated by the parameter overlap, which is given as a fraction of the spool position.

The spool position sp is a function of the input signal spoolpos:

sp = SO(f,d,discrete(spoolpos))

where SO is a second order transfer function to model the spool dynamics. The function is characterized by the bandwidth (f) and damping (d). The model acts as a directional valve (a valve which is either open or closed) because the input signal spoolpos is rounded to 0 or 1:

spoolpos < 0.5 => 0

spoolpos >= 0.5 => 1

The pressure at both ports has a lower limit which is equal to the vapour pressure. Therefore the actual equations used in this component are: